1. Field of the Invention
The invention relates to well production control systems, and more particularly, to an electrically actuated downhole valve system.
2. History of the Prior Art
In the operation of petroleum production wells, it is necessary to provide valves located within the production equipment down in a borehole for the control of various functions in the well. For example, in the operation of a gas lift well, it is necessary to selectively introduce the flow of high pressure gas to the tubing of a well in order to clear the accumulated borehole fluids from within the well and allow the flow of fluids from the production zone of the producing formation into the well tubing and to the surface for collection. Periodically, a mixture of oil and water collects in the bottom of the well casing and tubing in the region of the producing formation and obstructs the flow of gases to the surface. In a "gas lift" well completion high pressure gas from an external source is injected into the well in order to lift the borehole fluids collected in the well tubing to the surface to "clear" the well and allow the free flow of production fluids to the surface. This injection of gas into the well requires the operation of a valve controlling that injection gas flow known as a gas lift valve. Gas lift valves are conventionally normally closed restricting the flow of injection gas from the casing into the tubing and are opened to allow the flow of inject gas in response to either a preselected pressure condition or control from the surface. Generally such surface controlled valves are hydraulically operated. By controlling the flow of a hydraulic fluid from the surface, a poppet valve is actuated to control the flow of fluid into the gas lift valve. The valve is moved from a closed to an open position for as long as necessary to effect the flow of the lift gas. Such valves are also position instable, that is upon interruption of the hydraulic control pressure, the gas lift valve returns to its normally closed configuration.
A difficulty inherent in the use of gas lift valves which are either full open or closed is that gas lift production completions are a closed fluid system which are highly elastic in nature due to the compressibility of the fluids and the frequently large depth of the wells. For this reason, and especially in the case of dual completion wells, the flow of injected gas through a full open gas lift valve may produce vibrations at a harmonic frequency of the closed system and thereby create resonant oscillations in the system generating extremely large and destructive forces within the production equipment. Gas lift valves of a particular size aperture positioned at a point of resonance within the well completion(s) may have to be replaced in order for the system to be operable.
While electrically controlled gas lift valves are also available, for example as shown in U.S. Pat. No. 3,427,989, assigned to the assignee of the present invention, they include the disadvantages of other gas lift valve which are position instable and which operate based upon either full open or full closed conditions.
Another application of downhole fluid control valves within a production well is that of chemical injection. In some wells, it becomes necessary to inject a flow of chemicals into the borehole in order to treat either the well production equipment or the formation surrounding the borehole. The introduction of chemicals through a downhole valve capable of only full open or full closed condition does not allow precise control over the quantity of chemicals injected into the well.
Another application of downhole flow control valves is that of a dual completion gas lift operation in a well. By varying the orifice size of the gas injection valve the differential pressure drop across the gas lift valve can be controlled so that the pressure of the gas inside each string of tubing at the injection valve can be matched with the needs of that particular formation. However, flow control valves capable of only full open or closed configurations contribute to imprecise control over the pressure drop. In addition, such systems also suffer from potential resonance due to oscillations generated by flow through the valve which may necessitate tuning the system in some fashion or replacement of the valve in order for the system to be operable.
As mentioned above, prior art flow control valves for downhole applications, such as gas lift valves, include a number of inherent disadvantages. A first of these is having a single size flow orifice in the open condition which may produce resonant oscillations resulting in destructive effects within the well. A second disadvantage is that of being capable of assuming only a full open or full closed position which requires the shuttling of the valve between these two positions at high pressures and results in tremendous wear and tear on the valves. Such wear requires frequent maintenance and/or replacement of the valves which is extremely expensive. Hydraulically actuated downhole flow control valves also include certain inherent disadvantages as a result of their long hydraulic control lines which result in a delay in the application of control signals to a downhole device. In addition, the use of hydraulic fluids to control valves will not allow transmission of telemetry data from downhole monitors to controls at the surface.
To overcome some of these objections of present downhole flow control valve systems, it would be extremely helpful to be able to provide a downhole valve in which the orifice size of the valve is adjustable through a range of values. This would enable systems such as gas lift systems which are susceptible to resonant oscillation, to be detuned by adjusting the size of the orifice so that the system is no longer resonant. Changing the size of the valve flow control orifice allows the spontaneous generation of oscillations in a closed elastic fluid system to be damped and prevents the necessity of replacing the valve. In addition, such a variable orifice valve would allow much greater control over the quantity and rate of injection of fluids into the well. In particular, more precise control over the flow of injection gas into a dual lift gas lift well completion would allow continuous control of the injection pressure in both strings of tubing from a common annulus. This permits controls of production pressures and flow rates within the wells and result in more efficient production from the well.
Another desirable characteristic of a downhole flow control valve system would be that of position stability of the flow control orifice. That is, it would be highly useful to be able to set a flow control valve at a particular orifice and to have it remain at that same orifice size until selectively changed to a different size. Position stability is preferable in the absence of any control signals to the valve so that applied power is only necessary to change the orifice from one size to another. Prior art valves which are either open or closed, generally return to the closed state in the absence of control power.
Another large advantage which would be highly desirable in downhole flow control valve systems is that of an accurate system for monitoring not only the orifice size of the valve but also the pressures and flow rates within the production system in order to obtain desired production parameters within the well. For example, it would be advantageous to be able to select a particular bottom hole pressure and then control the size of the orifice of the valve in order to obtain that selected value of bottom hole pressure. Similarly, it would be desirable to be able to select a given flow rate and then control the size of the orifice of the valve in order to obtain and hold that particular flow rate of production flow from the well. Such systems require a reliable means for both sending data uphole from the vicinity of the valve as well as processing that data and then actively controlling the size of the flow control orifice of the valve in order to obtain the desired results, as monitored by the system. One implementation might include an indicator system for encoding and sending data to the surface related to valve orifice position and downhole pressure and flow information as well as a reliable system for sending signals downhole to selectively adjust the position of the valve.
The flow control valve system of the present invention incorporates many of these desired features of a valve system and allows the adjustment of a variable orifice size valve by means of signals from the surface and then the maintenance of that orifice size in a position stable configuration until additional signals are sent to change that orifice size. The system also has provisions for monitoring a plurality of parameters down in the well and then controlling the position of the valve in order to effectuate desired changes and/or maintenance in those parameter values.